CD audio discs contain at least a first session formatted in compliance with the well known ‘Red Book’, also known as Standard 908 of the International Electrotechnical Commission (IEC) entitled ‘Compact Disc Digital Audio System’ (Geneva, Switzerland, 1987).
CD-ROMs contain one or more sessions formatted in compliance with the ‘Yellow Book’ standard and normally its extension ‘System Description CD-ROM XA’, which includes a data retrieval structure based on ISO 9660. The ‘Yellow Book’ standard incorporates the ‘Red Book’ standard, so that CD data drives can play audio data as well as read non-audio data. Multisession CD-R and CD-RW discs comply with the Orange Book standard.
In this specification, a distinction will be made between ‘audio CD players’, which need only be able to read CDs complying with the Red Book standard, and ‘CD-ROM drives’, which are able to read CD's complying with the Yellow Book standards, and normally the Orange Book standard as well. Audio CD players are generally stand-alone devices having no other functionality than audio reproduction, although they may be integrated with other devices. CD-ROM drives are peripherals for general-purpose computers or other similar devices which are able to load and run application programs selected by the user.
The advent of recordable CDs (CD-R) has made it generally easy and inexpensive to make unauthorised copies of audio CDs and CD-ROMs; for example by copying the entire contents of an audio CD to a computer hard disc and then writing this to a CD-R. The potential loss of revenue to recording companies from such activities is considerable, and indeed its impact has already been felt. Consequently, there is a need to prevent such unauthorised copying.
Red Book compliant CDs contain a great deal of information which is not necessary purely for playing audio tracks and which is ignored by most audio-only players. Hence, one approach to preventing unauthorised copying has been to deviate from Red Book compliance in a way that is ignored by audio CD players, but causes an error in a CD-ROM drive.
Examples of this approach are described in WO 00/74053, in which selected control data is rendered inaccurate or incorrect; the Table of Contents (TOC) may identify the audio tracks as data tracks, or may incorrectly identify the position of the Lead-Out. WO 02/075735, published after the priority date of the present application, discloses identifying the first session as CD-ROM data when it actually contains audio tracks. EP 1239472, published after the priority date of the present application, also discloses the idea of identifying audio tracks as data tracks in the TOC.
However, a CD copying application running on a computer to which a CD-ROM drive is connected may be programmed to ignore inaccurate or incorrect control information, such as inaccurate TOC entries.
The techniques mentioned above are intended to prevent reading of audio tracks by a CD-ROM drive, and thereby prevent storage of the tracks on a computer and subsequent recording or distribution on some other medium. However, some users like to play audio CDs on their computers. Hence, there is also a need to provide audio tracks in such a way that they can be played on a CD-ROM drive but cannot be re-recorded or distributed.
One solution to this problem has been to provide the audio tracks in a first session which cannot be read by a CD-ROM drive, and to provide an encrypted version of the audio tracks in a second session. The encrypted tracks can only be decrypted and played by a player program which plays the audio data by converting it to audio signals.
However, in systems which use a universal encryption key or set of keys, all content encoded with the universal key(s) can be decrypted once the key(s) are discovered. If different keys are used for each carrier, some means must be devised for distributing the key appropriate to each carrier.